The continued growth in urban population has led to high-density housing close to airports and highways. This has increased the exposure of the population to noise from a variety of sources, increasing the need to provide better sound insulation for the homes. For homes close to airports and highway, windows constitute the primary path through which noise enters a home. Therefore, window improvements provide the most satisfaction to home dwellers. According to many research results, the development of double-glazed windows with embedded active control systems can be an effective approach to reduce noise impact on homes.
One great challenge for an active noise control system for windows is the need for the actuators to be transparent. One approach that has been investigated by other researchers is to place loudspeakers on the sides of the cavity of double-glazed windows as secondary sources. However, this cavity control approach is not effective in controlling the panel radiation-dominated sound. Another approach is to use a small voice-coil actuator to vibrate the glass panel itself to generate the canceling sound. Although significant reduction in noise transmission is possible at the location of actuator, global noise cancellation over the entire panel with a single point actuator can be achieved only when the length of the panel is less than one-fifth of the sound wavelength in the air (e.g., 0.14×0.14 m2 for frequencies up to 500 Hz). Such a small panel is not practical for a real window application. Using multiple voice coil actuators is also not practical, since several actuators on a window pane would again destroy the aesthetics of the window. There is a need for transparent speakers that can provide distributed canceling sound over the entire surface of a large sized glass panel. The need of transparence for the windows application poses a great challenge to the development of such speakers.
Several research groups have investigated different methods for the development of thin film acoustic actuators. One prior method uses an electroacoustic loudspeaker that uses the electrostrictive response of a polymer thin film. Over 80 dB sound pressure level can be produced from the “bubble” elements of such loudspeakers. However, the high resonant frequency (about 1500 Hz), the experienced harmonic distortion, and required high driving electric field (25 V/μm) will prohibit its use from most applications. Piezoelectric effect is another mechanism that can be employed to fabricate loudspeakers. Among the piezoelectric polymers, polyvinylidene fluoride (PVDF) has been mostly studied due to its strong piezoelectric effect. Recently, PVDF has been investigated for the active noise and vibration control, either being used as sensor, actuator, or both. However, the need of transparency for the electrodes still poses a challenge.
Transparent conductive thin films electrodes are also widely used for liquid crystal displays (LCDs), touch screens, solar cells and flexible displays. Due to high electrical conductivity and high optical transparency, indium tin oxide (ITO) thin films are often used in these applications. Typically, ITO thin films need to be deposited or post annealed at high temperatures to achieve an optimal combination of electrical and optical properties, which is much higher than the Curie temperature of PVDF. PVDF will lose desired piezoelectric properties at such high temperatures. Another shortcoming of ITO films prepared by such conventional methods is their brittleness. A 2% strain will make the films crack and thus lose conductivity. Antimony tin oxide (ATO) is a material similar to ITO, but has a greatly reduced conductivity. Other films have also been tried, but either lack conductivity or desired optical properties.
Transparent thin film acoustic transducers also have many other diverse applications. For instance, thin film speakers can work as transparent compact and lightweight general-purpose flat-panel loudspeakers. Attaching transparent thin film speakers onto the surface of windows, computer screens, posters, and touch panels can enable them to be “speaker-integrated” devices. This provides displays that may be able to talk, and touch pads, and windows that can serve as invisible speakers, windows that can serve as media centers, and other applications. Further, transparent thin film microphones can work as invisible sound monitors for military applications.